Various types of sensors exhibiting magnetoresistive characteristics are known and implemented in systems for the reading of information signals recorded in a magnetic medium such as tapes, drums and diskettes. These sensors typically comprise a block made of a ferromagnetic alloy exhibiting high magnetoresistance. A recording medium passing in close proximity to such a sensor causes variations in the magnetic field at the position of the read head, and hence variations of the electrical resistance of the magnetoresistive sensor. The alloys most frequently used are nickel-based ferromagnetic alloys such as NiCo or NiFe (Permalloy) which have high magnetoresistance but which, at ambient temperatures, give a relative variation in resistance of only a few percent at the relevant (average) magnetic fields (.about.50 G) accessible to those particular sensors.
Recently however, magnetoresistive sensors have been described exhibiting a form of magnetoresistance generally known as "spin-valve" (SV) magnetoresistance, in which the change in resistance of the sensor is attributed to the spin-dependent transmission of conduction electrons between the magnetic layers of the sensor and the accompanying spin-dependent scattering at the layer interfaces. In such a sensor, the magnetoresistance is observed to vary as the cosine of the angle between the magnetizations of the layers and is dependent of the direction of current flow through the sensor. Yet while such sensors do exhibit a magnetoresistance that, for selected combinations of materials, is greater in magnitude than that exhibited by the alloy magnetoresistors (AMR), they too suffer from having a relatively small variation in magnetoresistance at ambient temperatures.
More recently, magnetoresistive sensors for reading/writing information signals stored on a magnetic medium and constructed from undoped MCT have been described in copending applications Ser. No. 08/396,819 filed on Mar. 2, 1995, now U.S. Pat. No. 5,646,051 and Ser. No. 08/435,254 filed on May 5, 1995, now U.S. Pat. No. 5,646,051, each assigned to the same assignee as the instant application and incorporated herein by reference. Such sensors offer the advantage of a pronounced magnetoresistance
Importantly, magnetoresistive sensors are normally fabricated with a thin overlayer of hard magnetic material to provide a bias field required for optimum performance. The bias field can serve a number of purposes including: a) pre-aligning the spins in a polycrystalline sensor such as permalloy, b) shifting the operation of the detector to a region in which the field dependence of the resistance, R(H), is increased relative to that at H=0, c) shifting to a region of increased linearity and/or d) in the case of detectors with a symmetric response where R(H)=R(-H), providing the necessary asymmetry to sense the field direction.
As shown in some of the prior art for magnetoresistive sensors incorporating MCT as the magnetoresistive element, a bias field is required to introduce an asymmetry in the field response. Therefore, the fabrication, design and construction of an MCT field sensor could be greatly simplified if the material was self-biasing, especially if the self-biasing field could be controlled in the fabrication process. One such example of a self-biasing nonmagnetic giant magnetoresistance sensor was described in an article by S. A. Solin, T. Thio, J. W. Bennett, D. R. Hines, M. Kawano, N. Oda and M. Sano, entitled "Self-biasing Nonmagnetic Giant Magnetoresistance Sensor" that appeared in Applied Physics Letters, Vol. 69, Dec. 23, 1996. There, the authors describe room temperature giant magnetoresistance exhibiting a zero-field offset of as much as 350 G at room temperature. The resultant asymmetry in the field dependence of the GMR constitutes a self-biasing effect. Given this promising approach for developing sensors and its potential applicability to computer storage devices, additional improvements in the magnetoresistive material itself is desirable. Consequently a continuing need exists in the art for improved magnetoresistive materials and methods for fabricating such self-biasing, nonmagnetic giant magnetoresistance sensors.